1. Field of the Invention
This invention relates to a magnet base drill unit. More particularly, this invention relates to a magnet base drill unit which provided with a safety device that stops driving of the drill unit magnetically adhered onto a workpiece surface by an electromagnetic base thereof when the drill unit begins to slide on the workpiece surface.
2. Description of the Prior Art
A magnet base drill unit has so far been proposed, in which a drill unit with an automatic feeder and a magnet base are integrated, and the drill unit is attracted and fixed to a workpiece such as of metal using the magnet base to perform a hole cutting work.
The attraction force of the magnet in the prior art drill unit, is set so that it fully resists the cutting resistance caused by the hole cutting work of the drill unit and the feed load of the drill cutter, but by an unexpected cause, for instance, an excessive load occurring at the drill tip when cutting is started, or increase in the load due to the built-up edge formed on the drill cutter, and/or further, chips filling between the drill cutter and the workpiece during the hole cutting work, the load provided on the spindle of the drill motor and/or in the automatic feeder of the drill cutter may happen to exceed the attraction force of the magnet base.
In such case, the magnet base drill unit is subjected to a rotating force centering around the drill cutter and slides on the workpiece, and it can uncontrollably rotate while it is not attracted to the workpiece, or it can fall down violently. Thus, in such a case, at least the rotation of the drill cutter should be stopped instantaneously.
For this, in the U.S. Pat. No. 4261673 issued to Everett D. Hougen, for instance, magnet base drill unit is proposed which comprises a microswitch connected in series to the electric circuit of the drill motor, and a means for driving the switch which is constructed so as to turn on the switch when the attraction of the drill unit is normal and turn off it when the drill unit slides.
The above described switch drive means, as shown in FIGS. 3-5 of the specification, comprises a plunger which is movable in the direction perpendicular to the workpiece and is biased bY a spring toward the workpiece, a finger having a pointed end and rotatably supported by a pivot pin at the workpiece side of the plunger, and a means for biasing the finger so that the pointed end faces the workpiece side, wherein the pointed end of the finger somewhat bites the workpiece when the attraction of the drill unit is normal, thereby preventing the drill from sliding.
Accordingly, when the magnet base drill unit slides, the finger pivots about the pointed end and the plunger is driven toward the workpiece, so that the above-mentioned switch is turned off and the rotation of the drill is stopped.
The above described prior art had the following problems. The aforementioned switch turns off by its drive means when the magnet base drill unit is made to slide, whereby the operation of the drill unit is stopped, but the drive means of the switch is required to be constructed so that the switch maintains an off state even after the sliding has ceased, which makes the construction of the drive means relatively complex. As a result, the magnet base drill unit becomes large-sized and the manufacture thereof is cumbersome and expensive.
Moreover, since the finger is adapted to pivot about the pivot pin, if the pivot pin is positioned so that its axis intersects the center axis of the drill cutter for instance, the switch drive means operates well when the magnet base drill unit slides about the drill, but it may not operate if the magnet base drill unit slides so that the cutting position itself of the drill shifts on the work face.